Effective and low-cost energy storage devices are the subject of extensive research efforts. Supercapacitors are one technology under consideration as an improved alternative to over typical rechargeable batteries.
There are two classes of supercapacitors, distinguished by their mechanism of operation. The first, referred to as electrochemical double layer capacitors (EDLCs), include two electrodes that are mechanically separated from one another while also electrically connected by an electrolyte (a mixture of positive and negative ions within a solvent). Applying potential between the positive and negative electrodes results in attraction of negative and positive ions to the positive and negative electrodes, respectively. As a result, an electrical double layer is generated at each electrode. The double layer includes two different charge layers, one formed in the surface of each electrode and the other charge layer formed from opposite polarity ions within the electrolyte. The two charge layers are separated by a layer of polarized solvent molecules, producing a static electric field within the solvent separation layer that stores electrical charge. The amount of charge stored per unit voltage within the supercapacitor is primarily a function of the electrode size. As no chemical change takes place within the electrode or electrolyte, the ability to charge and discharge the supercapacitor is theoretically unlimited. The second class of supercapacitors, referred to as pseudocapacitors or redox capacitors, stores charge electrochemically by electron charge transfer between the electrode and electrolyte. This charge storage is achieved by electroabsorption, reduction-oxidation (redox) reactions and intercalation. Of the two supercapacitor designs, EDLCs are preferred due to the absence of electrochemical reactions.
Supercapacitor electrodes are typically formed from porous materials. The porosity provides improved access to charges from the electrolyte, which in turn increases the effective electrochemical surface area of the electrode available for formation of the double layer. Porous carbon is typically employed as an electrode material due to its low cost, high specific surface area, and easily accessed ordered pore channels. However, porous carbon electrodes suffer from poor electrical conductivity and mechanical flexibility, as well as relatively low specific capacitance and cycling stability.
Recently, electrodes formed from graphene and carbon nanotubes have been proposed. However, these structures suffer from numerous limitations, including efficiency, structural integrity, electrical conductivity, reduced porosity, and limited ability to process in bulk.
Accordingly, there exists a continued need for improved electrodes for use in energy storage devices such as supercapacitors.